Is there an easy way to find out if it is my operating system or my hard drive that is causing my computer to run slowly?

I pay for a Geek Squad plan at Best Buy, but they want to charge me 139 to fix my pc if it is a problem with my OS. If it is my hard drive they will replace it at no charge.

Is there a way for me to do a diagnostic test without using the Geeks? Is there a way for me to tell if it is the OS or the hard drive? Otherwise, I have to pay my money up front.

This is what I am really getting at: Is there an easier way to fix the problem if it is the OS, or should I shell out the bucks and call it a day?